1. Field of the Invention
This invention relates to an exposure apparatus and method suitable for use in exposing a pattern of a large area onto a photosensitive substrate as in a case where, for example, a liquid crystal panel or the like is manufactured by the photolithography process.
2. Related Background Art
Recently, considerable attention has been devoted to a scanning type exposure apparatus in which, in order to manufacture, for example, a large liquid crystal panel (liquid crystal display element), a semiconductive element of a large area or the like, a slit-like (rectangular, arcuate or otherwise shaped) illuminated area is formed on a mask (such as a photomask or a reticle), the mask is scanned in the direction of the shorter side of the illuminated area, and a plate having photoresist applied thereto (such as a glass plate or a semiconductor wafer) is scanned in synchronism in an exposure area conjugate with the illuminated area, whereby patterns on the mask are successively transferred onto the plate.
In such a scanning type exposure apparatus, as well as in an exposure apparatus wherein exposure is effected onto each shot area on a plate in a stationary state, auto focusing and auto levelling mechanisms are necessary for accurately adjusting the exposed surface of the plate to the imaging plane (best focus plane) of a projection optical system so that the patterns on the mask may be transferred onto the plate with high resolution. In the case of the scanning exposure type, the mask and plate are scanned during exposure and therefore, the auto focusing and auto levelling mechanisms tend to become complicated as compared with a case where exposure is effected in a stationary state.
FIG. 7 of the accompanying drawings shows an example of the scanning type exposure apparatus according to the prior art. Exposure light from a light source 1 such as a mercury lamp is condensed by an elliptical mirror 2 and enters an illuminating optical system 3. The exposure light IL emerging from the illuminating optical system 3 forms a slit-like illuminated area 5 of uniform illuminance distribution on the pattern forming surface of a mask 4. The mask 4 is held on the upper stage 6a of a scanning stage 6, and a plate 8 is held on a lower stage 6b with a Z levelling stage 7 interposed therebetween. The scanning stage 6 is driven on a base 11 in .+-.X directions, and between the upper stage 6a and the lower stage 6b, a projection optical system 9 for projecting the erect positive image of a mask pattern at one-to-one magnification is fixedly disposed relative to the base 11. The image of the pattern in the illuminated area 5 is projected onto a slit-like exposure field 10 on the plate 8.
In this case, the image of the mask pattern in the exposure field 10 is an erect positive image and therefore, by the scanning stage 6 being driven in X direction to thereby scan the mask 4 and the plate 8 as a unit in X direction, the patterns on the mask 4 are successively transferred onto the plate 8.
Also, a focus detecting system of the oblique incidence type comprising a light transmitting optical system 12a and a light receiving optical system 12b is provided to detect the position of the surface (exposed surface) of the plate 8 in the exposure field 10 in a direction (Z direction) parallel to the optical axis of the projection optical system 9. In this focus detecting system, for example, a slit pattern image is obliquely projected from the light transmitting optical system 12a onto the plate 8 in the central portion of the exposure field 10, and the reflected light from the plate 8 enters the light receiving optical system 12b and the slit pattern image in the light receiving optical system 12b is re-imaged. When the plate 8 is displaced in Z direction, the re-imaged slit pattern image deviates laterally.
The position of the re-imaged image when the exposed surface of the plate 8 is coincident with the imaging plane of the projection optical system 9 is memorized in advance as a reference position, and a focus signal corresponding to the amount of deviation of the re-imaged image from the reference position is produced by the light receiving optical system 12b. The amount of deviation of the projection point (measurement point) of the slit pattern image in Z direction relative to the imaging plane is detected from the focus signal. The Z levelling stage 7 is driven to continuously adjust the position of the plate 8 in Z direction so that the amount of deviation in Z direction may be negated when the mask 4 and plate 8 are scanned, whereby scanning exposure has been effected with auto focusing.
Further in some cases, two measurement points have been provided on the plate 8. When there are two measurement points like this, a focus detecting system of the same construction as the focus detecting system comprising the light transmitting optical system 12a and the light receiving optical system 12b has been further provided in the direction perpendicular to the plane of the drawing sheet of FIG. 7 and the amounts of positional deviation in Z direction at the two measurement points have been detected. At this time, the difference between the amounts of positional deviation at the two measurement points indicates the inclination of the exposed surface of the plate 8 in a plane perpendicular to the scanning direction and therefore, the Z levelling stage 7 has been driven to effect the correction of the inclination by the auto levelling system.
As described above, in the scanning type exposure apparatus according to the prior art, the amount of positional deviation of the plate 8 in Z direction has been detected at one or two points in the central portion of the exposure field 10 of the projection optical system 9 and in a direction perpendicular to the scanning direction, and focusing or levelling has been effected on the basis of this amount of positional deviation.
In the prior art, however, when the flatness of the mask 4 is bad or when the position of the mask 4 fluctuates in Z direction depending on the accuracy of the running of the scanning stage 6, the position of the imaging plane of the projection optical system 9 changes, and this has led to the inconvenience that simply by correcting the position of the plate 8, it is impossible to adjust the exposed surface of the plate 8 to the imaging plane.
Even if the flatness of the mask 4 is good and the accuracy of the running of the scanning stage 6 is good, when the exposed surface of the plate 8 is a curved surface, the shape of the exposed surface could not be accurately found if the amount of positional deviation in Z direction is detected at two or less measurement points in the direction perpendicular to the scanning direction, and this has resulted in the inconvenience that it is impossible to adjust the average plane of the exposed surface of the plate 8 in the exposure field 10 to the imaging plane.